The present invention relates to a stencil printer for printing an image on a paper by supplying ink from the inner periphery of a drum to the paper via a stencil or master wrapped around the drum and perforated in accordance with image data. More particularly, the present invention relates to an improvement in the structure of the drum and the structure of the stencil.
A stencil printer includes a thermal head having heating elements. The heating elements selectively generate heat to perforate a thermosensitive stencil in accordance with image data and thereby form an image in the stencil. The perforated stencil, or master, is wrapped around a drum made up of a porous support and a mesh screen of resin or metal. Ink is supplied from the inner periphery of the drum by a supply member while a paper is continuously pressed against the master by a press roller or similar pressing member. As a result, the ink oozes out via the pores of the drum and the perforations of the master, thereby printing an image on the paper.
The drum for the above application is disclosed in, for example, Japanese Utility Model Publication No. 59-229 and Japanese Patent Publication No. 63-59393. The drum taught in Publication No. 63-59393 has a porous support and a plurality of mesh screens wrapped around the support. The outermost mesh screen is implemented by members of about 250 mesh (pitch of about 100 .mu.m) and having a diameter of about 40 .mu.m. Pores for passing ink therethrough are sized 60 .mu.m to 70 .mu.m square each.
The heating elements have customarily been sized about 40 .mu.m square each and provided with a resolution of 400 dots per inch (dpi). The heating elements are forms a perforation of substantially the same size as itself in the stencil.
The conventional stencil has a laminate structure comprising a film of polyester of similar thermosensitive resin and as thin as about 1 .mu.m to 2 .mu.m, and a porous flexible support implemented as a layer of synthetic fibers or Japanese paper or a mixture layer of Japanese paper fibers and synthetic fibers. The kind of stencil has the following problems. In portions where the Japanese paper fibers are entangled together, ink is obstructed and prevented from being transferred to a paper. As a result, fiber marks appear in the resulting image. For example, a fiber pattern appears in a solid image portion having a substantial area, or thin lines become blurred. Another problem is that when the used master is discarded, the ink deposited thereon is also discarded. This is wasteful from the resource standpoint.
It is a common practice with the stencil printer to use sparingly volatile oil ink or emulsion ink in which oil wraps water. With this kind of ink, when the printer is operated after a long time of interruption, there can be obviated an occurrence that a number of papers are simply wasted due to the evaporation of ink from the support of the drum and mesh screen or the absorption of ink by the porous support of the stencil.
However, the problem with the ink of the kind described is that along period of time is necessary for it to infiltrate into the paper and fully dry. In a continuous print mode, when a paper or printing is laid on the previous printing whose ink is still wet, the ink is transferred from latter to the rear of the former. This is particularly true with a solid image portion to which a great amount of ink is deposited.
In light of the above, it has been proposed to reduce the thickness of the support of the stencil or omit it in order to reduce the fiber marks and the amount of ink to be discarded. However, the outermost mesh screen of the drum has openings of substantially the same size as perforations to be formed in the stencil. Hence, even the above proposed scheme scarcely obviates the transfer of the ink from the underlying printing to the rear of the overlying printing. Further, when the support of the stencil is omitted, the stencil or master contacts the protruding portions of the mesh screen. As a result, when the paper is pressed against the master during printing, the stencil suffers from wear and, therefore, holes due to friction. This causes the ink to smear the paper.
To eliminate the undesirable ink transfer, previously mentioned Publication No. 63-59393 teaches a drum having a hollow cylindrical support having a number of pores, an inner screen layer surrounding the support, and an outer screen layer surrounding the inner layer. The mesh value is sequentially increased from the support to the outer screen layer in order to reduce the size of ink passages, i.e., to reduce the amount of ink to be drawn out from the drum. This kind of structure is also disclosed in Japanese Utility Model Publication No. 5-41026 in which the mesh value is sequentially decreased from the inner screen layer to the outer screen layer. However, because the openings of the outer mesh screen have substantially the same size as the perforations of the stencil, the ink does not break off sharply and is transferred to a paper in a great amount. This also results in the transfer of the ink from the underlying printing to the rear of the overlying printing.